Apoptosis of autoreactive T cells has been recognized as an important mechanism of immune homeostasis in autoimmune PNS diseases. To examine whether T cells are induced to undergo apoptosis by macrophages (Mphi) via reactive oxygen species (ROS) neuritogenic P2-specific T line cells were exposed to peritoneal Mphi of Lewis rats in vitro. ROS production was stimulated by phorbol myristate acetate (PMA) and inhibited by catalase. Flow cytometric analysis of apoptosis as measured by TUNEL technique revealed that 5-15% of all P2-cells were apoptotic, if cultured alone. This percentage increased slightly, but did not exceed 20% when P2-cells were cocultivated with PMA-stimulated Mphi in a two-chamber culture plate separated by a cell-impermeable membrane. Direct cocultivation caused apoptotic cell death of more than 40% of P2-cells, which was completely abrogated by catalase. These results suggest a requirement for close cell-cell contact for apoptosis induction of T cells via Mphi-derived ROS. Thus Mphi may contribute to termination of inflammation in vivo through the release of highly reactive oxygen species that mediate apoptotic cell death of autoaggressive T lymphocytes.